Let’s say you’re a prospective buyer touring an older home that you suspect has some weatherization issues. What if you could verify your hunch by literally seeing cold air seeping under doors or cooling walls where insulation is missing? And what if you could do this on the spot using a smartphone?
Long associated with million-dollar, airborne cameras that look like upside-down droids mounted on helicopters, airplanes, and ships, thermal cameras are currently small enough to fit inside a cell phone. Now, consumers can utilize lower-resolution thermal imagers in a variety of practical applications, from home repairs and electrical inspections to looking for a lost child at night.
Earlier this year, FLIR Systems released a pocket-sized thermal imager called FLIR One that attaches like a case to the iPhone 5 and 5s (see Figure 1). Using the iPhone’s LCD screen, the imager measures and displays invisible heat energy (also called thermal energy) instead of visible light. With standard settings, warmer areas (like the inside of a house) will appear brighter on screen, while cooler areas (cold air sneaking in) will appear dark.
In this article, we will explore the emergence of compact, low-cost thermal imagers and their current impact on the development of commercial products equipped with thermal imaging.
Early Thermal Imagers
Thermal technology has been around for a century, but the first thermal imaging applications did not emerge until shortly after World War II, when the US Air Force developed a rudimentary reconnaissance thermal camera mounted in the belly of cargo planes and bombers. The first commercial application of a thermal imager appeared in 1965, when the Swedish company Agema built the first infrared scanning camera for power line inspection.
In order to sense faint thermal signatures, the early thermal imagers cooled their internal detectors to cryogenic temperatures with liquid nitrogen. Later models achieved the same cooling with internal cryo-coolers. Today’s long-range thermal cameras use highly advanced versions of these coolers to maximize range performance. The thermal cameras are mostly used by the traditional government customers: the military and law enforcement agencies.
Uncooled thermal cameras operate without the need of additional cooling. A common detector design uses a microbolometer — a tiny vanadium oxide resistor with a large temperature coefficient. Changes in scene temperature cause changes in the bolometer temperature that are converted to electrical signals and processed into an image. Uncooled sensors work in the longwave infrared (LWIR) band, from 7 to 14 microns in wavelength, where terrestrial temperature targets emit most of their infrared energy.
The uncooled cameras are generally much less expensive to produce. The sensors can be manufactured in fewer steps, with higher yield and less expensive vacuum packaging. Because they are far less expensive than their cooled counterparts, uncooled thermal cameras have enabled broad commercialization of thermal imaging.
In the thermal camera world, resolution is the most significant element to improving image quality. The higher the thermal resolution, the more detail the camera captures. There are generally three resolution standards: 160 x 120 (low), 320 x 240 (medium), and 640 x 480 (high). Which resolution is most appropriate is really a matter of the final application. A soldier using a thermal rifle scope in combat, for example, is going to require exceptional detail, while a thermal imager with a lower resolution will be perfectly suitable for predictive maintenance in a factory.
The micro thermal camera (MTC) inside FLIR One demonstrates a miniaturization of uncooled thermal camera cores. Named Lepton®, the uncooled imager is produced using wafer-level technology, where the thermal sensor, lens, and supporting electronics package all fit on a single chip (see Figure 2).
Lepton has a resolution of 80 x 60 pixels, far less than its more powerful cooled siblings. Nonetheless, just as a smartphone can now accomplish what used to take a room full of mainframe computers, MTCs can be manufactured at a high volume and equip new mobile devices and camera systems with thermal imaging capabilities.
Creating the compact thermal camera required several proprietary technologies, including a wafer-level detector packaging, wafer-level micro-optics, and a custom integrated circuit that supports all camera functions on a single, integrated, low-power chip.
The MTC operates on low voltages found in standard mobile phones, fits into a standard 32-pin Molex socket, and uses control and serial video interfaces that are compatible with phone standards. The serial video interface is MIPI compatible and features a D-PHY transmitter to send serial video data and clocks to the host. The video is accessible either via the MIPI interface or as packetized video using SPI. The control interface is similar to the I2C protocol. Designers supply standard mobile industry voltages and a clock to get thermal images through mobile industry standard interfaces.